Method of separating gas mixtures by adsorption



April 6, 1965 METHOD OF SEPAARATING GAS `MIXTURES BY ADSORPTION FiledSept. 16. 1960 J. J. COLLINS ETAL 2 Sheets-Sheet 1 ,G G39 GHVOD "NN, e

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f L l 'i o nl m #n a 2 F i @a g M DE N D Q N N 3 JNVENToRs 12.1 s JOHNJ. COLLINS A T TORNEY April 6, 1965 METHOD OF SEPARATING GAS MIXTURES BYADSORPTION Filed Sept. 16. 1960 J. J. coLLlNs ETAL 3,176,445

2 Sheets-Sheet 2 yanoverhead or demethanizer off-gas.

United States Patent O This invention relates to an improved process forrecovering substantially pure product gas from a gas mixture containingsuch product gas and impurity gases,

' and more specifically to a process for removing carbon dioxideimpurity from a gas mixture containing ethylene and carbon dioxide bycontacting such mixture with a crystalline zeolitic molecular sievematerial.

Ethylene is a basic chemical for many organic synthesis processes.Several types of ethylene-formation processes are in use today. By farthe most common is that which employs some sort of externally firedtubular coil by means of which the feedstock is thermally cracked so asto provide ethylene in a mixture of hydrogen, hydrocarbons and otherimpurities such as sulfur, carbon dioxideand water. Basically anethylene producing unit isrrnade up of three parts; the ethyleneformation section; the section wherein auxiliary purification of thevethylene takes place and the separation unit wherein the ethylene isseparated from other hydrocarbons such as 'the methane and lighterhydrocarbons. The separation of ethylene from methane usually is carriedout in a refrigerated column called a demethanizer unit from which themethane and lighter hydrocarbon is passed as This oil-gas consistsprimarily of hydrogen. and methane with a small f 'amount of ethylene.The ethylene content of demethanizer off-gas ranges from about 0.2 toabout l2 mol percent withy the typical content being between 0.5 to 3.0mol percent. The end-use of the ethylene, whether such be for themanufacture; of organic chemicals such as ethylalcohol or polymerproducts such as polyethylene, is independent rof the source or methodof manufacture of the ethylene as long as the purity of the ethylene issatisfactory for the particular end-use.

This invention in its broader aspects provides a method of increasingthe product recovery in a separation cycle yutilized for.` thepurification of a product gas stream ,product gas, as for example theethylene formation process where there is readily available ademethanizer overhead Vgas which contains from about 0.2 to about 12 molpercent with the usual content being 0.5 to 3.0 mol percent ethylenewhich heretofore was often being burned as fuel andwasted. As a resultof the novel method of recovering ethylene which was usually discardedthe invention provides unusually high recovery of ethylene product whenseparating impurities such as carbon dioxide from ethylene. f

Accordingly a principal object of this invention is to provide a novelseparation process for removing impurity from a product gas formed in aprocess where there is available a lay-product gas stream containing aminor r`amount of product gas.

ice.

Still another object is to provide a method for purifying lower weightolefins formed from a process wherein there is also produced aby-product gas stream containing minor amounts of such olens.

Another object is to provide a method for separating carbon dioxide fromethylene wherein increased yields of ethylene are realized.

Briefly the above objects are realized according to the preferred formof the present invention by utilizing at least three crystallinezeolitic molecular sieve iilled chambers alternating consecutively inthe steps of (a) adsorption, (b) blowdown and repressurization, (c)thermal desorption withk cocurrent flow of purge gas and (d) cooling andpartial preloading with product gas, (e) blowdown and (f)repressurization with ethylene feed.

The step of cooling and pre-loading may be performed simultaneously whenusing a three bed system. In four bed system the cooling and pre-loadingsteps are done step wise. Selection of a four bed system over a threebed system depends on (l) purity of demethanizer overhead (2) ethyleneconcentration-demethanizer overhead (3) quantity of demethanizeroverhead available.

in order that one skilled in the gasseparation art may more fullyunderstand the inventive concept disclosed herein the followingdescriptions and discussion will be directed to the separation of carbondioxide from ethylene but it is to be understood that the basic conceptmay also be utilized in a propylene formation unit where there isreadily available a by-product gas stream containing minor amounts ofpropylene.

rhe inventive process described hereinafter is particularly adaptable toa process for separatingY CO2 from ethylene wherein the CO2 contentofthe impure ethylene feed stream ranges from about parts per million toabout 3 mol percent with a typical content of 0.2 to 1 mol percent.

Some advantages of the inventive process are:

(l) The use of demethanizer olf-gas to cool the molecular sieve bedafter desorption effects a recovery of ethylene from the olf-gas whichis normally burned as fuel. The recovered ethylene is ultimately addedto the purified product. Additionally in a three bed system theadsorption of ethylene from the demethanizer off-gas during the coolingstep effects a partial pre-loading of adsorbate on the zeoliticmolecular sieve thereby reducing the heating effect when theethylene-rich feed stream contacts the sieve bed on the adsorption step.ln a four bed system the cooling and preloading steps are performedstepwise. The demethanizer off-gas used to cool the molecular sieve bedafter desorption is previously used to pre-load another molecular sievebed with thel result that ethylene is removed before the demethanizeroff-gas is used as a cooling gas. Adsorption of the ethylene from thedemethanizer off-gas during the preloading step effects at least apartial pre-loading of adsorbate on the zeolitic molecular sieve therebyreducing the heating eifect when the ethylene rich feed stream contactsthe molecular sieve bed on the adsorption step.

Pre-loading the molecular sieve bed with ethylene results in a signicantimprovement in the operation of the purification system. This isdue to:

(l) Rmuced bed-temperature rise during repressurization of the bed withethylene feed just prior to adsorption. This will result in a highercarbon dioxide operating loading during the adsorption step of thepurification.

(il) Reduced ethylene product contamination due to the presence ofadsorbed impurities loaded on the bed during cooling. Since the bed isnow loaded with ethylene at the start of adsorption, there is verylittle adsorption on the bed of ethane, methane, and other components ofthe ethylene-bearing cooling gas, which would normally of operatingconditions. /r Y i V(IV.) .Reduced deactivation rateandincreasedadsorbhave moderately high bed-loadings. but are less strongly` adsorbedthan ethylene.

(III) Reduced carbon dioxide cVoncentratic)ns inthe ethylene productduring adsorption. z Since the .tempera-V ture vrise-at the start ofadsorption is vminir'nized, the resid ual carbondioxide left at theeffluent endof the Vbedduring regenerationwill have a lower equilibrium'partialnpressure in theethylene product. This results in-ralowerV CO2'concentration in the ethylene vproduct for any l given "set"` A entlife 'since thebed isnow exposedfto'feedethylene atfv Y desorbate willbe rich in carbondioxide. Thisfdesorbed displacement desorptionof'ethylene kremaining inthe -down-stream part ofrthe'bed. Thus'theethylencis'eifectively desorbed at a lower temperature andthe accumula'-`tion .of'carblonaceous deposits through the polymerization arevadequately described v able synthetic zeolitic molecular sieves A, D,R, S, T, X,Y, and L. f Y

TheV preferred zeolitic r'nolecu-lansievesare rthose thatr inthechemical art.`y The suitinclude zeolites havepore sizes of atleast 4.6Angstrom units andinclude erionite, calciurn-rich chabazite, faujasite,Ythe synthetic zeolites, "gS, Y, L, T, and thedivalent'cationaexclianged zeolites A, D, `and Re The larger pore sizepermits'more rapid Vadsorptionfand: desorptionof the,v carbon dioxidelofrmolecules leading to faster operating cycles inthe process of thisinvention; m

The 'pore size of the zeolitic molecular sieves may be y varied byVemploying.,different,metal cations.Y For example, sodiumV zeolitefA hasay porel isize ofl about 4l lAngstrom units whereas calcium zeolite Ahas a-po're size of about 5 yAngstrom units, when rcalciumcations havebeen exchanged for at least about40 percent of the sodium carbon'dioxidethen o'wsthrough the .bed effecting a f ,and/Y or cracking ismaterially, reduced. This reducesrthe rate .at which lthe adsorptivecapacityfof` the 'molecular' yIt is to be'understoodtha't'the expressionpores-Vize,y

' Vas used herein refersto theapparent pore size, as dis@ `tinguishedfrom .the effective pore diameter. Thei appar#y Aent pore size maybedeiinedfas the maximum critical sievev whose composition maybeexpressed, in terms of,

sieve would be reduced. j l

dimension of the Vmolecular. speciesV which is Vadsorbed by the.zeoliticmolecular sieve :in question, Vunder normal conditions. 'Maximumcritical dimensions may be defined as the diameter ofthe smallestVVcylinderI whichrwill accations, so that the latter Vwould be suitableas ya preferred material for use'in the Vpresentrinvention. Y

il YZeolite'Ais a crystalline `zeolitic molecular sieveV which Y maybe'v represented byv tlfre'forrnula;v

` 1.002M c =A12 O;1',85io.5siozyn2o wherein M represents a metal,1n isthe valenceof vM, and y may have any value up to about 6.-lhelas-synthesized zeolite A contains primarily sodiumions and isdesignated sodiumlzeoliteA.` Zeolite A is describedrin more `detail inUS.' Patent No. 2,882,243, issued'. April 14, l1959.

Zeolitel T is a synthetic, crystalline zeolitic, molecular ."oxide moleratios as follows: 1

as the free diameter'of appropriate silicate ring inthe i ZeolitestructureThe apparent; pore ,size'forfa' given zeolitic molecular sievewill ynormallybe larger than the effective pore diameter. Y f a Theterm,zeolite, in general, refersv to a group of natare, however,significant dilferencesbetween the various lsynthetic-,and naturalmaterialsV inl chemical composition, Y crystal structure and physicalproperties'such as X-ray y powderdiffraction patterns.y Y

The structure of 'crystalline'.'zeolite molecular sieves urallyoccurring and synthetic hydrated metal "'aluminof' 'n silicates, many ofwhich are crystalline in structure.;fI'herey negative electrovalence of`tetrahedra containing` aluminum a isbalanced by the inclusion Withinzthecrystal of cations,Y

for example, alkali'metahand alkalineearth metal ionsglsuchas sodium,potassium,fcalcium and magnesium-ions.' Y VOne catlon may be exchangedfor another by ion-exchange techniques. f l The zeolites'may beactivated 'by driving Off srbstanttar" rzeolite.

of about 10 angstrom units. AvZeolite X, its X-ray dif-r fractionpattern, its properties,and methods for its preparation ar'edescribed indetail in U.S. Patent No. 2,882,244 issued April 14, 1959.1.

igiioparxNag) (f1fxnczoiinno,zssiossiozynzo wherein xfis'lanyyaluefromabout 0.1.fto about. 0.8 and yis any'Y value from about zero toabout 8. Further characterization-ofzeolite r'l isbytmeans ofQX-r'aydiffraction techniques asdescribcd in U.S.fPatentjNo. 2,950,952,

issued'August 30, 1960'. 1 i v 'Zeolite- X is a synthetic crystallinezeolitic molecular sieve which `may be represented-by Ithe formula:v a

wherein VMrepre-sents a metalpparticularly alkali yand'alkalinevearthvmetala'n is the valence A ofvM, and y may 1 have anylvalueup to abourtr8,depending on the identity ofxM-andthe'degree/ofhydration of the crystalline Sodium zeolite X hasanyapparent pore size zeom'v is descnbedY and daimdfi Us. parental-Vplication, Serial No. V7280547iiled. April` 14, Y1958,l and- U.S;lpatent application VSerialNo.=862,062,f1ed concurrently/herewith, bothinthe'namefof D. W; Breek, as

.",well asSerial No. 109,487 filed May 12, 1961 and issued ly all ofthewater of hydratiom The space remaining in the crystals afteractivation is availableV foradsorption ofpadsorbatefmolecules. Thisspaceis available. for ad'- a sorption of molecules4 having asize,fshape1and energy which permits entyof'the adsorbate moleculesintothe pores of the molecularlsieves. Y.

The zeolitesoccui as agglomerates of 'fine crystals orVr are synthesizedas fine powders and are preferably tableted orV pelletized forlarge-scale adsorption uses.,V V-Pelletizing methods are known Whicharevery satisfactory becauseV the sorptive character of thezeolitefbothwith regard.

to selectivity and capacity, remains essentially unchanged.

VApril 21,1964 as Us. Paintmgrro.3,130,007;4 the am mentionedapplications are now abandoned. 1 Y

,Zeolite L is described andgclaimedjin UgS. patent applicationSerialNo..711,565,1led,Januaryr28, v1958, in

the namesrof vD.W. Brecka'nd NVA. Aear'a. r.This ap- .'plication wasabandoned in favor of continuation-inpart application Serial No.-214,479 filed August 3, 1962.

Zeolit'eVV D is described and'claimed in ULS. patent application SerialNo'. l680,383,iled `August 26,l 17957, in

the name of1R. M.iMi1t0n.' This applicaties was abandonedin favor ofcontinuation-impart application'Serial Y"No.'27r3,5'49. f

' Theaadsorbent is employedY inwthe'v formi of pellets,

.which are moreconveniently `utilized inapacked chambers than'theunpelletized finely crystalline zeolitic ymolecular Among'the naturallyoccurring zeoliticmolecular sieves sultable for use in thepresent-invention are chabazite and erionite, mordenite, and faujasite.The Ileltral materials 'sieveS.

The'commercially available pellets contain about 12.0%V clay binder. i al l In 4the preferred .embodiment iof the invention there is provided afour -bed adsorption system as rshown in FIG. 1. Each bed containsmolecular sieve material having a pore size of at least 4.6 angstromunits. This pore size is preferred because an adsorption front will beestablished more easily if the molecules of the carbon dioxide impurityhave an easy ingress and egress from the inner adsorption area of thesieve.

The preferred embodiment is most admirably suited lto a separation ofcarbon dioxide from ethylene in an ethylene formation process where thedemethanizer off- `gas is contaminated with carbon dioxide and acetyleneand where the demethanizer off-gas lacks suicient ethylene for efficientpre-loading of the sieve bed.

In this separation cycle carbon dioxide containing ethylene stream iscontacted with the first adsorption zone containing zeolitic molecularsieve material as an adsorption step thereby adsorbing at least most ofthe carbon dioxide and :some of the ethylene product gas.

The adsorption pressure for the process ranges from atmospheric to 1500p.s.i.a.with typical feed pressures between 50 and 600 p.s.i.a. The feedtemperature can range from about 40 F. to 250 F. with typical valuesbetween 50 F. and 100 F. If temperatures below 40 F. are used liquidphases may be encountered and desorption is made more diicult. While iftemperatures much above 250 F. are employed the working loading isimpractically low and the deactivation rate may become excessive.Sub-atmospheric pressure engenders the problems of vacuum techniqueswith possibility of air leaks into the system. Also mass liow ratesinvolve excessive velocities in the equipment. Above 1500 p.s.i.\a.,special equipment is required and feed holdup in the bed voids becomeunreasonably high.

As the carbon dioxide laden feed gas stream contacts the zeoliticmolecular sieve bed the more strongly a-ttracted polar carbon dioxidemolecule is adsorbed in favor of the ethylene molecule and a carbondioxide impurity adsorption front is established whereby the carbondioxide molecule continuously displaces most of the adsorbed ethylenemolecule. An impurity depleted product gas stream is discharged from thelirst adsorption zone.

At carbon dioxide breakthrough, that is when the gas stream leaving thefirst adsorption zone has not more than the maximum allowable carbondioxide content the adsorption step is preferably stopped. At this pointmost of the ethylene adsorbed and not displaced by the carbon dioxideadsorption front is located toward the effluent end of the bed.Continuing purication to carbon dioxide breakthrough in the ethyleneproduct rather than operating on a xed cycle time results in less totalethylene holdup in the adsorption zone at the end of the adsorptionstep. By running to carbon dioxide breakthrough, the maximum amount ofimpurity is loaded onto the bed at the end of such adsorption step. Byrunning on a fixed cycle time, the bed is over-designed until theadsorbent has been cycled for such length of time that the adsorbent hasdeactivated to the point where carbon dioxide breakthrough is occurringat the end of the adsorption stroke. The excess capacity is taken up byethylene, about one-half of which is subsequently lost duringregeneration. The other. one-half is recovered during blowdown.

A controlled cocurrent depressurization step follows the adsorptionstep. It has been discovered that when the volume percent of carbondioxide is greater than ,about 0.2 percent the use of a cocurrentdepressurization step after the adsorption stroke becomes increasinglyimportant in improving the yield of ethylene. A larger percentage of thetotal ethylene coadsorbed with the CO2 on the adsorbent and entrapped inthe void space in the crystalline zeolitic molecular sieve beds isrecovered. By using the technique of cocurrent depressurization asdescribed in detail in copending application Serial No. 60,709, nowabandoned, and its continuation-in-part application Serial No. 221,033tiled September 4, 1962,

vention. The present process for recovering pure product gas from a gasmixture containing such product gasvand impurity gases comprises aspecific sequence of steps, and in a preferred embodiment one of thesesteps `is cocurrent depressurization.

vFrom the standpoint of both maximum recovery and purity, a cocurrentblowdown in the depressurization step is preferred. A cocurrent blowdownresults in a higher ethylene recovery than a countercurrent blowdown fortwo reasons. First, the eluent end of the bed, which has the highestethylene loading, ends up at the lowest final pressure When the blowdownis cocurrent. The net result will be desorption of the maximum amount ofethylene for a given final blowdown pressure. Secondly, by blowing downcocurrently, most,if not all of the carbon dioxide desorbed from thefeed end of the bed will be readsorbed at the eiiluent end of the bedthus promoting ethylene desorption by displacement.

When the blowdown is cocurrent, the carbon dioxide content of theethylene efuent will generally vary from 5 ppm. at the start to 150p.p.m. at-16 p.s.i.a. final pressure. When the blowdown iscountercurrent, the carbon dioxide content of the ethylene eliiuent'willgenerally vary from about its concentration in the feed to 70 volumepercent.

As the CO2 content of the ethylene feed decreases below 0.2 percent,and/ or the pressure of the feed ethylene decreases, the advantages of acocurrent blowdown (described above) are not as marked. In such Cases, acountercurrent blowdown may be selected. -From the standpoint ofmaximizing the C2H4 recovery during the blowdown, it may be advantageousto continue the blowdown to a pressure below ambient; in this case avacuum pump or compression is needed. In no case will it be advantageousto reduce the pressure during the blowdown below the CO2 partialpressure in the feed. The ethylene collected during the first stage of acontrolled cocurrent depressurization is stored in a surge tank. Thisstored ethylene is later used to partially preload the adsorber bed onthe pre-loading step in the process cycle. This ethylene is used to addmore of such gas to the demethanizer off-gas which is the primary gasused to pre-load the adsorber bed.

The next step in the cycle is the regeneration step which is carried onin bed B after the blowdown. Cocurrent desorption is used for this step,that is, hot purge gas is caused to iiow through the bed in the samedirection as the gas feed stream. The hot purge gas consists, in thisembodiment, of demethanizer overhead which has been used earlier forcooling, plus a natural gas stream. The regeneration step may employ anyinert purge gas such as methane, nitrogen, hydrogen, etc. It is notnecessary to use the eiiiuent dernethanizer gas from the bed on coolingalthough this may reduce the size of the heater necessary to heat thegas for purging purposes to a temperature desirable. Further ifsuflicient demethanizer olf-gas is available to conduct the hot gasregeneration in the time period allocated to this step, inert gas neednot be added to the stream entering the heater.

The next steps of cooling and pre-loading are conducted by use of thedemethanizer overhead. The demethanizer gas is rst purified in a guardbed to remove acetylene and carbon dioxide and -then used to preload thecool adsorber bed D. Ethylene from the earlier mentioned surge tank isused to make up any additional ethylene required for pre-loading thebed. The efliuent from this pre-loading step is used to cool adsorberbed C. This method of operation prevents the loss of ethylene from thepre-loading step inasmuch as preloading is conducted past ethylenebreakthrough. A pair of guard beds alternating in adsorption anddesorption are used in the demethanizer overhead stream to `25. `feed tobe introduced through conduit 24 to bed D which beds.V Of course,'ifthereare not impuritiesintthe de'-V methanizer overhead theguard bedsare notl'necessary." After preloadingthe bed is blown down atacontrolled This is done tof remove the demethaniz'erover V rate.

' tactcd vip/ithV the iirstl adsorption zone' containing zeoliticmolecular sieveY material as 4antadsorp'tion step'. thereby headentrapped inthe rbed voids and toremove compo-l ,Y

Y' `nents of demethanizer overhead (e.g., methane, ethane) that rarecoadsorbed Vwith the ethylene on the molecular sieve bed. The bed-isthen repressurized Vwith feed and Y placed back on adsorption. Y p Y pThe following ldetailed description of an example of the preferredinventive separation cycle is presentedkso Y `that those skilled in theart may more clearlyl under stand the manner of practicing theinvention. 'Y Y' Referring to FIG. l, in this embodiment acarbonAdioxide vimpurity containing ethylene -stream is introduced at 22,000lb./hr. and at about 75V F. and 510 p.s.i.g. through conduit 10 andvalve 12 to Yzeoliticmolec-Y ular sieve bed A. A carbon dioxidedepletedethyleneY gasstream is discharged from bed A through branchconduit 16, valve 18 and conduit 2t). At the end of ther adsorptionstroke -valves 12 and 1S are closed Vand bed` A is switched to bed Bposition. Controlled blowdown is commenced by' opening valvef19 anddecreasing the pressure in bed B froml about 510 tor250 psig., asaresult of the ydecreasein pressure, cocurrent depressuriza` tiontechnique ethylene is recovered from ,bed B and passes through conduitZ1 and valve ,23vto surge tank S; Then bed B is depressurized .furtherin steps from about 250 to about 120 p.s.i.g. and then from 120 p.s.i.g.

to atmospheric by closing valve 23 and opening valve Previously closedvalveY 22 Vis opened Vpermitting was previously preloaded. While bed Ais on adsorption, bed B is on blowdown and regeneration, bed C is.

by providingrdemethanizer overhead at about 50 F. and

ene product gas. Asthe CO2-laden feed gas stream conv tacts the zeoliticmolecular 'siev'exbed the more strongly minimize coking of thesievematerial'on'the desorption step,V cocurrent desorption is employedIn this embodiment of the novel separation cycle the cocurrentdesorption. step andthe cooling and preloading step are performed inseries. These steps are accomplished by providing as a purge gas ademethanizer'overhead gas to the Vthe feed gas at the adsorption stroke.

third adsorption'zone which has previously been reactivated or desorbedand whichv is at about 500 F. The demethanizer gas used for cooling isat a temperature substantially less than 500 F. preferably close to thatof As the cooling gas is passed Ythrough the bed and the bedcools, itwill have an increasing adsorption capacity. As the ethylene-containingcooling gas passes through the bed being cooled `most of the ethylene ofthe demethaniz'er gas will beadsorbed on the cool portieri of the bed..An equilibrium condition existshetween the ethylene in the cooling gasand the ethylene loaded 0n the bed.y For example, if the pressure of thecooling gas (assume essentially all hydrogen) is 150 p.s.i.a. and theethylene concentration is 1.5%

. the equilibrium ethylene loading on the cool bed (-100 about 125p.s.i.g.V through valve ,14,l guard bed A', .conduit ethylene fromV thepurified demethaniz'er. stream isvr adsorbed by, bed D thus pre-loadingsaid bed prior to the introduction of the feed stream thereto. Eethylenedepleted demethanizer gas leaves bed D through valve 26 :and passesthrough conduit 27, heat exchanger 28, conl. there isV insulicientdemethanizer available to conduct the 'Y hot gas regeneration in thetimeperiod allocated.` Thus natural gas is provided through conduit 34to heater H.

The combined demethanizer and Vnaturalfgas streamis heated to about 550=F. andthen is passed through conduit 35 and valve 36 to bed B wherecocurrent regeneraction takes place. The regeneration gas leaves bed Bthroughxrvalve 38 Vand then passesthrough condut'37-up'1 v to guard bedB'. This effluent from adsorber bed B Yis used to desorb guard bed B. f

. By periodically switching theV flows to Vand fromy each of saidadsorption zones each bed is caused to cyclically go through each step,inV the above described process.

In another embodiment Yof Vthe'invention. there is prol vided a threebed adsorption system as shown in FIG. 2.

Y15 and valve 17 to bed D which has previouslyV completed a Vcoolingstroke and is at about 100 F. The

' 100 lb. molecular sieve] IF.) is -5.5 1b.*C2H4/100lb7. molecular sievetype 5A. v[It the pressure of thecooling gas is -150 p.s.i.a. (assumeessentially all CH4), and the ethylene concentration -is 71.5%,l theequilibrium ethylene Yloading is -4.Y2 lb`./

The cooling gas leavingthe'third Yadsorption zone may be (a) sent tofuel, (b) used somewhere else inthe plant, Y(c) passed Vthrougha'm'ake-up heater and used to purge the molecular sieve bedthat isregenerating. In'this case the gas leaving the thirdV adsorption zone 1spassed'through a heater and then caused to ow in a Y cocurrent directionthrough a second adsorption zone to ydesorb orvregenerate said zone. Inthe regeneration or desorption step, the molecular sieve material'at theinlet Aend of the bed is contacted with the now hot purge gas. As thetemperature increases Vthe more strongly held carbon dioxide will bedesorbed and a carbon dioxide desorption front will` be established atone end of the bed. .The carbon dioxide from this desorption front willbe :'readsorbed by theV cooler molecular sieve in a movingcarbonldroxrde adsorption front further down the bed Each zone containsa bed of molecular sieve material hav; Y

ing an apparent pore size of at least aboutI 4 angstromV units. A poresize of at least 4.6 angstrom units is preferred because the CO impurityadsorption front will be l established more easily if the molecules ofsuchCOz'impurity have an yeasy ingressV and egress from'the inneradsorption area of the sieve. Y 'Y Y In this embodiment CO2-laden feedgas Vstream is con- Vthrough. the bed by the hot' purge gas. As suchcarbon dioxide adsorption front ofthecarbon dioxide pulse progressesthrough the bed,1most of the residual ethylene inthe bed is continuallydesorbed in favor of the more vstr ongly'held4 carbon dioxide. Theresult is that contact of the hotv purge vgas, withthe ethylene isminimized since the carbon dioxideV acts as a bufer and cokingv issubstantiallyrreduced. Y p

The'above describedfprocess is especiallyl useful for separating carbondioxidefromethylene kwhen the de- Ymethanizer doesnot contain suchimpurities. as CO2 Yand C2H2 which must'be removed with aguard lbedsince their presence in the C21-,I4 product, is not desirable. Y Y Atthe start ofregeneration, the gas leaving the'moleck Yular sieve bedwillrbe rich in C2H4. It'may be possible use this ethylene. 7 5 f torecycle. this gas to `some point inthe ethylene plant `to n l Theethylene vcontent of the eliluent gas atatihe early stage ofregeneration will be very high (e.g., 10% of a blowdown is used, 30 lb.percent if no blowdown is used).

The following is an example of the embodiment of the invention shown inFlG. 2. Under the conditions in the example below a net increase in theethylene yield of about 2.0% over the method omitting the use ofdemethanizer overhead.

In this example the feed gas stream contained about 0.7 mol percent ofcarbon dioxide. The temperature may be from 40 to 150 F. ln this casethe feed was at about 75 F. The feed ystream was at a pressure of S10`p.s.i.g. and was fed at the rate of 22,000 lb./hr. to adsorber' A astheV adsorption step. The adsorption step is .terminated in bed A whenthe content of carbon dioxide inthe product stream is about parts permillion. While bed A is an adsorption, beds B and C are beingregenerated and cooled and preloaded respectively. Demethanizer off-gascontaining about 2% ethylene at a temperature of about 50 F. and apressure of about 140 p.s.i.g. is fed to bed C which has previouslycompleted the regeneration step and is at about 500 F. Bed C is cooledand the demethanizer off-gas is heated while ethylene is removed fromsuch gas by adsorption step. The

. ethylene depleted off-gas is further heated in a heating means to atemperature of about 500 F. and then passed kin a'cocurrent directiondown through bed B which has previously completed an adsorption step andis at a teinperature of about 75 F. The bed temperature is raised toabout 500 F. at which point desorption is sufficiently complete toachieve satisfactory operation on return to adsorption; however if inthe timing of the overall process cycle there remains time, additionalpurging may be done to further reduce the residual carbon dioxide in thebed.

In a cyclic manner each of the beds alternate consecutively onadsorption, regeneration7 and cooling and preloading. b

rReferring more specifica-ily to a FIG. 2 carbon dioxide impuritycontaining ethylene stream is introduced through conduit 60, valve 61and branch conduit 62 to zeolitic molecular sieve bed A. A carbondioxide depleted ethylene gas stream is discharged from bed A throughbranch conduit 63, valve 64 and conduit 65. At the end of the adsorptionstroke valve 6l is closed and previously closed valve 65y is openedpermitting feed to be introduced through conduits 6o and 67 to bed Cwhich was previously cooled and pre-loaded. While bed A is onadsorption, beds B and C are on regeneration and cooling and pre-`loading respectively. This is accomplished by providing demethanizeroverhead gas at about 50 F. and 140 p.s.i.g. through conduit 68, valve69 and conduit 67 to bed C 'which has previously completed aregeneration step and vregeneratebed B. The hot purge gas leaves theheater H through conduit 73 and enters the bed B through valve 74 andconduit 75 to commence the regeneration of said bed which has previouslycompleted adsorption. Bed B which is at a temperature of about 75 F. isdepressurized and lraised to a temperature of about 500 F. The purge gasleaves bed B through branch conduit 76, valve '77 and lconduit 78.

While bed A is adsorbing and beds B and C are being regenerated andcooled and pre-loaded valves 79, 80, Si

yand 82 associated with bed A; valves 83, 84, S5 and 86 associated withbed B and valves 65, 87, 83 and 89 associated with bed C are all closed.It is to be understood that the appropriate valves will be opened toeach bed for the corresponding step in the cycle.

Although the inventive concept has been described in detai-l referringto a carbon dioxide from ethylene separation and referring to thepreferred embodiments shown in the drawings it is contemplated thatmodification of the method may be made and that some lfeatures may beemployed without others all within the spirit and scope of theinvention.

What is claimed is:

l. A process for purifying an ethylene gas stream which comprisesproviding a fixed bed of crystalline zeolitic molecular ysieve materialhaving pore sizes of atr least 4 angstrom units; providing an ethylenefeed gas stream containing an admixture of carbon dioxide impurity inethylene gas product; contacting such stream with said bed as anadsorption step by lintroducing Said feed gas stream Iat the inlet endthereof to adsorb at least part of said carbon dioxide impurities andpart of said ethylene product gas in said bed of crystalline zeoliticmolecular sieve material, discharging a carbon dioxide impurity depletedethylene product gas stream from the opposite end of said bed;establishing a carbon dioxide impurity adsorption front at said inletend; progressively moving such front longitudinally through said fixedbed toward the opposite end thereof thereby displacing said ethyleneproduct gas with carbon dioxide impurity in said crystalline zeoliticmolecular sieve material as the result of the movement of said carbondioxide impurity adsorption front; terminating the introduction of saidfeed gas stream to said bed to complete the adsorption step; as aregeneration step introducing a hot purge gas having a temperaturesubstantially higher than the temperature at which the adsorption steptakes place to the inlet end of said bed and flowing said purge gas inthe same direction as said rfeed gas to regenerate such bed; as acooling step providing a cooling gas at the inlet end of said bed at atemperature less than that at which regeneration takes place and fiowingsaid cooling gas in the same direction as said feed and purge gases; andas preloading step providing at the inlet end of said bed a gascontaining at least part of said ethylene product gas and fiowing suchgas in the same direction as said feed, purge and cooling gases toadsorb said product gas in such zone thereby partially preloading saidbed with a product gas prior to the beginning of the adsorption stepwhereby a net increase in the yield of ethylene product gas is realized.

2. A process for continuously purifying a gas stream which comprisesproviding a feed gas stream containing an admixture of impurity in gasproduct; providing atleast four adsorption zones, each having an inletend and a discharge end and containing therein a bed of crystallinezeolitic molecular sieve material having pore sizes of at least 4angstrom units, said bed having voids between said zeolitic molecularsieve material; introducing said Vfeed gas stream to a first adsorptionzone inlet end and contacting such stream with the first adsorption zonebed at a rst pressure as an adsorption step to adsorb at least part ofsaid impurity and part of said gas product in said zeolitic molecularsieve material and trapping part of said gas product in said voids;discharging an impurity depleted product gas stream fromY the firstadsorption zone discharge end; establishing -an impurity adsorptionfront at said first adsorption zone inlet end; progressively moving suchfront longitudinally through said first adsorption zone toward saidfirst adsorption zone discharge end to a predetermined point within saidfirst adsorption zone thereby displacing most of said adsorbed productgas with impurity in said zeolitic molecular sieve material as theresult of the movement of said impurity adsorption front; terminatingthe introduction of said feed gas stream to said adsorption zone inletend to complete the adsorption step in said first adsorption zone; as acocurrent depressurization step controllably reducing said firstpressure in said first adsorption zone to a second lower pressurethrough said first adsorption zone discharge end thereby further movingsaid impurity adsorption front toward the discharge end of said firstadsorption zone so as ,d to remove the trapped product gas 'fronrthe'`voids of' saidj Y* rst ladsorption zone; as a regeneration stepdesorbing a second Iadsorption; zone having previously completed the icocurrent depressurization step;VV said regeneration ,step

Y being accomplished byintroducing a hotpurge gas-to the inlet-end ofsaid second radsorption zone forrilow` intheY Y rsame direction assaidjfeedig'as to regenerate saidfzone;

lcompleted aregeneration step; said regeneration step yand saidcoolingand preloading step being'accomplished `subst antiallyVsimultaneouslyby passing ysaid cooling gas 'through said.' third"adsorption zone Vfromfthe inlet end to the 'discharge end thereofVajnd'liny the same'direction as vsaid feed gasto cool said thirdadsorption zone and simultaneouslyl adsorb saidproduct Agasthert-,bypartially preas a cooling step providing a cooling gasV stream at the."inlet end of athird adsorption zone having'previously l completed aregeneration step and iloWing-saidv cooling gas through the third zoneinthe Asame directionVV as the feed and purge gases; and as a preloadingstep providing at the inlet end -of a fourth'y adsorption zone a gas,stream f higher than that of the. adsorption step thereby formingcontaining a product gas fraction and flowing the'pre'load- Y ing gas inthe same direction as the feed, purge andi-cool-V ing gases to adsorb insuch fourth zone 'at least part ofr said product gas fraction therebypartially preloadingsaid fourth zone with product gas prior to saidadsorption step insaid fourth zone; discharging a product `depleted'gas' lstream from saidV fourth adsorptionzone dischargeend andintroducing such gas stream to saidthird"adsorptionzone inlet end as thecoolinggas streamV to cool such third'zone; periodically switchingtheilowbetWeen said adsorption zones soY that the adsorption steptakesplace inthe vzone that has'previously completed the preloadingstep; they cocurreut depressurization step takes' place 'in the zonethat has previously. Vcompleted the adsorption step; the cooling loadingsaid thirdzone-fwith said 'product gasy prior to said s adsorption stepin saidthird'vzone; then discharging the product-'depleted cooling gasAfrom Vsaid third adsorption Zone and'heating suchk gas toa temperaturesubstantially a purgegas;` introducing the purge gas to the inlet end ofsaid second adsorption zone as 's'aid'regeneration step and owing saidpurge; gas to thev discharge end in thev same "direction as the feed andcooling gases; periodically switching the tlovvs between-said adsorptionzones so that the adsorption step takes place in the zone that haspreviously completed a cooling and4 preloading step,l the `rek26Tgeneration step takes place in thelone that has previously completedthe` adsorptionr step,l and ,the cooling land preloading step 'takesplace in the zione that has'previously' ,completed the regenerationstep.

n 71A process according to claim 6 wherein the zeolitic step takes placein the zone that-has previouslyv completed Y ka regeneration Isteptheregeneration step v4takes place-in the zone that'has previouslycompleted the cocurrent depressurization step; and the preloadingstep-takes'place in p the zone thatY has previously completed theycooling step.

3. The process according to claim Zinwhichthe` impurity is carbondioxide `and the product gas -isethylenei v 4. A process according toclaim ZVWher'einthe crystal-k line vzeolitie molecular sieve materialhas a pore sizeof f at least about-4.6rangstromrunits. W f

5; VA process according to' clair'rn4, wherein the hot lpurge gasintroduced to said's'econd adsorptionY zone in-k Y let end establishesan impurity pulse consisting ofan im'- V vpurity desorption front-and animpurity adsorption front in the inlet end of said second adsorptionzone, progres-` sivelyV moving such pulse longitudinally throug'njsuchY'25frnolecular.y sieve materialisselected fromthefclass consisting ofthe naturallygloccurring lcrystalline molecularA ksieve eronit'e,.calciumrich' chabazite, faujasite; the syn- 'Vtheticr zeolites X,Y,"L',g T `and the divalent-cation exchanged zeolites `D andV Rli*'carbondioxde'in ethylene; providing at least three ad- {sorptionzones, each having lanlrinlet fand adischargeend andicontaining thereina bed ofcrystallinezeoliticV molecygular sieve, material having poresizes of at least 4 angstrom gunits, said bed havingvoids between saidzeolitic molecvular vsieve material; introducing said feed gas stream toa rst'adsorption vzone inlet end and contacting such second zone fromthe inlet'end-tofthe .discharge end thereof, thereby displacing theresidual of said 'adsorbed impurity product gas from the zone of theprogressing adsorption front 'of said impurity pulse.'l

6. A process for continuously-purifying yan,etliyleneiV gas stream whichcomprises'providinga feed gas stream `'containing an admixture ,ofcarbon dioxide impurity in ethylene gas product; providing'at yleastthree adsorption Y zones, each having an inlet end and a, discharge endand containing therein affixed bedof crystalline zeolitic molecularsieve material having pore sizes of at least 4Yangstro`m units;introducingsaidrfeedgas stream to a t'rst adsorption zoneinlet end andcontacting suchfstrearn Withlthe 'rst adsorption zone bed asA anadsorption step to adsorb atlleast part ofsaid carbon'dioxide impuritiesand `part of said ethylene product `gas VinV said zeoliticimole'cular'sieve material; discharging ja carbon dioxide impurity depletedethylene Aproduct'V gas stream from Ythe tirst" adsorption zonedischarge end;1establishinga carbon'dioxide impurity adsorption `frontat' s aid first. adsorptionV zoneinlet end; rprogressively moving suchfront longitudinally through sadmtirst adsorption. zone towardsaid'iirstj adsorption zone .dischargeendtherebyldisplacing said eth-rylene product with carbon dioxide impurity'inV said zeolitiomolecularsieve material asrthe result ofv they move.

ment of said impurityadsorption iront; terminating the introduction ofsaid feed gas stream to vsaid irsLadsorp-i 'tion zone; as a regenerationstep desorbing 'alsecond advsorption zone having previously completed anadsorp-r; y tion step; and as acooling and preloading' stepjproviding acooling-gas containing atleast portropf said ethylene product gas in athird adsorptionzonehaving previously `stream with. the irst adsorptionzone bed at a -trst pressure-as an adsorption step togadsorlb atleastpart' of Vsaid carbon dioxide andpart of fsaid ethylene in said zeoliticf molecularsieve material and trapping part of said ethyleneinl-saidvoids; discharging a carbon dioxide depleted fethylene gasstream from the iirst'adsorption yzonedischarge end; establishing acarbon dioxide adsorption front at said first VadsorptionV zionel inletendgvprogressively mov- 'fing'such' front.longitudinally'through saidrst adsorption zone toward said -iirst adsorption gone discharge end "toa .predetermined `point'within said tirstadsorption zone thereby'displacing 'mostof said adsorbedethylene gas With carbondioxidein-said'zeoliticmolecular sieve material asthe result4ofthem'ovementof said carbon dioxide adsorption front; terminatingftheintroduction of said feed gas streamlt'o said iirstadsorption Vzoneinlet end to complete theyadsorption step in said iirst adsorption zone;

- as ar cocurrent depressurization stepcontrollably reducing f said rstVpressure in said `first, adsorption'zone to a second lowerv pressurevthrough'said first adsorption'zone discharge end4 thereby furtherrmovingl said carbon dioxide Y 'adsorptionitr'ont toward the dischargeend ofv vsaid 4first adsorption zone'thereby-removing the trappedyethylene -gas from the voids of said iirst adsorption zone; as aregeneration step des'orbing a second adsorption zone having previouslycompleted 'a cocurrent depressurization step; and as a coolingandpreloadingY step providing agas stream containing anethylenelfractionin a third Iadsorptionl ZoneV having i, previously completed aregeneration step; said regeneration step' andfsaid cooling and preloadring step-l being accomplished substantiallyr simultaneously bypassing/acooling gas stream A.containing an ethylene gas fraction through saidthird adsorption zone from the inlet end to the discharge end thereofand in the same direction as said feed gas to cool said third adsorptionzone and simultaneously adsorb in such zone at least part of saidethylene gas fraction thereby partially preloading said third zone withethylene gas prior to said adsorption step in said third adsorption zoneand passing such gas through a heating means to raise the temperature ofsaid gas from about 75 F. to 500-550 F.; introducing the resulting hotpurge gas to the inlet end of said second adsorption zone and flowingsaid purge gas to the discharge end in the same direction as the feedand cooling gases to regenerate said zone; periodically switching theflows between said adsorption zones so that the adsorption step takesplace in the zone that has previously completed a cooling and preloadingstep, the cocurrent depressurization step takes place in the zone thathas previously completed an adsorption step, the desorption step takesplace in the zone that has previously completed the cocurrentdepressurization step, and the cooling and preloading step takes placein the zone that has previously completed the desorption step.

10. A process according to claim 9 wherein the hot purge gas introducedto said second adsorption zone inlet end establishes a carbon dioxideimpurity pulse consisting of a carbon dioxide impurity desorption frontand a carbon dioxide impurity adsorption front in the inlet end of saidsecond adsorption zone, progressively moving such pulse longitudinallythrough such second zone from the inlet end to the discharge endthereof, thereby displacing the residual of said adsorbed ethyleneproduct gas from the zone of the progressing carbon dioxide impurityadsorption front of said carbon dioxide impurity pulse.

11. A method for separating carbon dioxide impurity from ethyleneproduct gas in an ethylene formation process wherein a demethanizerolf-gas is readily available from other steps in the ethylene formationprocess, which comprises providing a feed gas stream containing anadmixture of carbon dioxide in ethylene; providing at least fouradsorption zones, each having an inlet end and a discharge end andcontaining therein a bed of crystalline zeolitic molecular sievematerial having pore sizes of at least four angstrom units, said bedhaving voids between said zeolitic molecular sieve material; introducingsaid feed gas stream to a first adsorption zone inlet end and contactingsuch stream with the `first adsorption zone bed at a first pressure asan adsorption step to adsorb at least part of said carbon dioxide andpart of said ethylene gas product in said zeolitic molecular sievematerial and trapping part of said ethylene gas product in said voids;discharging a carbon dioxide depleted product gas stream from the firstadsorption zone discharge end; establishing a carbon dioxide adsorptionfront at said iirst adsorption zone inlet end; progressively moving suchfront longitudinally through said first adsorption zone toward thedischarge end to a predetermined point within said first adsorption zonethereby displacing most of said adsorbed ethylene product gas withcarbon dioxide impurity in said zeolitic molecular sieve material as theresult of the movement of said carbon dioxide adsorption front;terminating the introduction of said feed gas stream to said adsorptionzone inlet end to complete the adsorption step in said first adsorptionzone; as a cocurrent depressurization step controllably reducing saidfirst pressure in said first adsorption zone to a lower pressure throughsaid irst adsorption zone discharge end thereby further moving saidcarbon dioxide adsorption front toward the discharge end of said firstadsorption zone so as to remove the trapped ethylene product gas fromthe voids of said rst adsorption zone; collecting at least part of theethylene product gas eliluent from the lirst adsorption zone dislidcharge end in a tank during such cocurrent depressurization step; as aregeneration step desorbing a second adsorption zone having previouslycompleted the cocurrent depressurization step by introducing a hot purgegas containing at least some of said demethanizer off-gas to the inletend of a second adsorption zone and owing said purge gas to thedischarge end in the same direction as said feed gas to regenerate saidzone; as a cooling step providing as a cooling gas stream thedemethanizer offgas to the inlet end of the third adsorption zone havingpreviously completed a regeneration step and flowing such gas to thedischarge end in the same direction as said feed and purge gases; and asa preloading step providing to the inlet end of a fourth adsorption zonesaid demethanizer off-gas together with ethylene product gas suppliedfrom said tank and -liowing said gas to the discharge end in the samedirection as said feed, purge and cooling gases to adsorb in such fourthzone at least part of said ethylene gas fraction thereby partiallypreloading said fourth zone with ethylene product gas prior to saidadsorption step in said fourth zone; passing said demethanizer off-gasthrough a guard bed to remove acetylene and carbon dioxide therefromprior to introducing said demethanizer off-gas to said fourth adsorptionzone; discharging an ethylene depleted demethanizer gas stream from saidfourth adsorption zone and introducing such ethylene depleteddemethanizer off-gas stream to said third adsorption zone as the coolinggas stream to cool such third zone; discharging said demethanizerolf-gas cooling stream from said adsorption zone and combining suchstream with an inert gas, passing such combined inert gas anddemethanizer off-gas stream through a heating means to raise thetemperature of such gas and then providing such hot combined gas streamto said second adsorption zone to perform the regeneration step therein.

12. A- process according to claim 1l wherein the feed gas stream isintroduced to said iirst adsorption zone at from about 50 to 100 F. andfrom about 50 to 600 p.s.1.a.

13. A process according to claim l1 wherein the feed gas stream issupplied to the first adsorption zone inlet end at the rate of 22,000lbs/hr.; at a iirst pressure of about 510 p.s.i.g.; and at a temperatureof about F.

14. A process according to claim 13 wherein the hot purge gas introducedto said second adsorption zone inlet end establishes a carbon dioxideimpurity pulse consisting of a carbon dioxide impurity desorption frontand a carbon dioxide impurity adsorption front in the inlet end of saidsecond adsorption zone, progressively moving such pulse longitudinallythrough such second zone from the inlet end to the discharge endthereof, thereby displacing the residual of said adsorbed ethyleneproduct gas from the zone of the progressing carbon dioxide impurityadsorption front of said carbon dioxide impurity pulse.

References Cited by the Examiner UNITED STATES PATENTS 2,519,874 8/50Berg 55-61 2,799,362 7/57 Miller 18S-114.2 2,901,519 8/59 Patterson183-114.2 2,944,627 7/ 60 Skarstrom 1835-1142 2,995,208 8/61 Hachmuth etal 55-180 X 3,085,379 4/ 63 Kiyonaga et al. 55-62 3,130,021 4/64 Milton55--33 FOREIGN PATENTS 555,482 4/58 Canada.

HARRY B. THORNTON, Primary Examiner.

EUGENE S. BLANCHARD, REUBEN FRIEDMAN, WALTER BERLOWITZ, Examiners.

1. A PROCESS FOR PURIFYING AN ETHYLENE GAS STREAM WHICH COMPRISESPROVIDING A FIXED BED OF CRYSTALLINE ZEOLITIC MOLECULAR SIEVE MATERIALHAVING PORE SIZES OF AT LEAST 4 ANGSTROM UNITS; PROVIDING AN ETHYLENEFED GAS STREAM CONTAINING AN ADMIXTURE OF CARBON DIOXIDE IMPURITY INETHYLENE GAS PRODUCT; CONTACTING SUCH STREAM WITH SAID BED AS ANADSORPTION STEP BY INTRODUCING SAID FEED GAS STREAM AT THE INLET ENDTHEREOF TO ADSORB AT LEAST PART OF SAID CARBON DIOXIDE IMPURITIES ANDPART OF SAID ETHYLENE PRODUCT GAS IN SAID BED OF CRYSTALLINE ZEOLITEMOLECULAR SIEVE MATERIAL, DISCHARGING A CARBON DIOXIDE IMPURITY DEPLETEDETHYLENE PRODUCT GAS STREAM FROM THE OPPOSITE END OF SAID BED;ESTABLISHING A CARBON DIOXIDE IMPURITY ABSORPTION FRONT AT SAID INLETEND; PROGRESSIVELY MOVING SUCH FRONT LONGITUDINALLY THROUGH SAID FIXEDBED TOWARD THE OPPOSITE END THEREOF THEREBY DISPLACING SAID ETHYLENEPRODUCT GAS WITH CARBON DIOXIDE IMPURITY IN SAID CRYSTALLINE ZEOLITEMOLECULAR SIEVE MATERIAL AS THE RESULT OF THE MOVEMENT OF SAID CARBONDIOXIDE IMPURITY ADSORPTION FRONT; TERMINATING THE INTRODUCTION OF SAIDFEED GAS STREAM TO SAID BED TO COMPLETE THE ADSORPTION STEP; AS AREGENERATED STEP INTRODUCING A HOT PURGE GAS HAVING A TEMPERATURESUBSTANTIALLY HIGHER THAN THE TEMPERATURE AT WHICH THE ABSORPTION STEPTAKES PLACE TO THE INLET END OF SAID BED AND FLOWING SAID PURGE GAS INTHE SAME DIRECTION AS SAID